Parametric modeling and configuration design methods are key technologies for mass customization in manufacturing. This thesis proposes a framework which parametrically models a machine tool assembly based on a design expert system. The concept of design unit, which is one level higher than the functional features, and the parametric modeling concept with functional features have been proposed. The domain knowledge in the knowledge-base has been mapped to the geometry of a CAD system. A design expert system to redesign assemblies of a machine tool has been implemented. This system consists of a commercial expert system shell, a design knowledge-base, a commercial CAD system, and an API program. The API program interfaces the expert system with the CAD system through the GUI.